High intensity discharge (HID) lamps are commonly used in large area lighting applications, due to their high energy efficiency and superb long life. The existing HID product range consists of mercury vapor (MV), high pressure sodium (HPS), and quartz metal halide (MH) lamps. In recent years, ceramic metal halide lamps (for example, Philips MasterColor® series) from 39 to 400W have entered the market place. Compared to the conventional HID lamps, these ceramic metal halide lamps display excellent initial color consistency, superb stability over life, (lumen maintenance >80%, color temperature shift <200K at 10,000hrs), high luminous efficacy of >90 lumens/watt and a lifetime of about 20,000 hours. These highly desirable characteristics are due to the high stability of the polycrystalline alumina (PCA) envelopes and a special mixture of salts, which emits a continuous-spectrum light radiation close to natural light. By adjusting the composition of salts used in said lamps, color temperatures of 3800–4500K, and a Color Rendering Index (CRI) of above 85 can be achieved.
One current design of MasterColor lamps utilizes a cylindrical PCA discharge tube with extended plugs for securing electrodes. The approximate aspect ratio of the PCA discharge tube, i.e. length/diameter, of the PCA body varies from 1 to 3 for lower wattages (39W–100W), and 3 to 10 for higher wattages (150W to 1000W). For the top of the line 400W and 1000W lamps, the lamp current is approximately 4.5A (ANSI standard) in steady state operation and is approximately 7–8A during warm up. The mount structure of the high wattage MasterColor lamps include a standard glass bulb with gas filling or vacuum, stem, connectors, getters, current carrying frame wire, and ignition aids such as UV enhancer or antenna. One of the designs for antenna is a conductive coil extending along the length of barrel and wrapped around the arc tube and around the extended plugs. The antenna coil reduces the breakdown voltage at which the fill gas ionizes by a capacitive coupling between the coil and the adjacent lead-in in the plug. When an AC voltage is applied across the electrodes, the antenna stimulates UV emission in the PCA, which in turn causes primary electrons to be emitted by the electrode. The presence of these primary electrons hastens ignition of a discharge in the fill gas.
When the said lamps are in steady-state operation, the gas pressure inside the discharge vessel ranges from 2 to 20 atmospheres. Therefore, it is possible that when the discharge vessel ruptures when the lamp is in operation, the fragments becomes energized and penetrate the outer glass bulb, posing risks to the environment. Therefore, the said lamps are subject to containment tests. By “containment” is meant the prevention of outer bulb damage caused by arc tube rupture. ANSI test protocol method for measurement for containment testing of quartz metal halide lamps is published as an appendix to American National Standard for method of measurement of metal halide lamps, ANSI C78.387-1995. The Mo coil antenna in the said lamps serves a dual function as containment protection and ignition.
Protected pulse-start metal halide lamps (with both low-wattage ceramic arc tubes and low/high wattage quartz arc tubes) use a quartz sleeve and often a Mo coil wrapped around the sleeve to contain particles within the outer bulb in the event of an arc tube rupture. These lamps do not require auxiliary antenna to aid the ignition process.
Other lamps such as HPS or sodium halide lamps use a refractory metal spiral to aid in starting and to inhibit sodium migration through the arc tube during operation. Representative of such uses are:
EP 0549056 which discloses a metal coil used for containment only and not for ignition. In addition, the coil is wrapped around a sleeve that surrounds the arc tube and is not wrapped around the arc tube itself.
U.S. Pat. No. 4,179,640 which discloses a coil used for ignition only in HPS lamps and not for containment. In addition, the coil is electrically connected to the frame wire and is not capacitively coupled.
U.S. Pat. No. 4,491,766 which discloses a coil used for ignition and inhibition of sodium migration and not for containment. In addition, the coil is electrically connected to the frame wire and is not capacitively coupled.
U.S. Pat. No. 4,950,938 discloses a metal screen and not a coil, the screen is used for containment only and not for ignition.
DE 2639276 discloses a high pressure sodium vapor lamp with a cylindrical mesh grid starting aid to permit lower operational voltages.
There is a need in the art for HID lamps of the ceramic metal halide type with power ranges of about 150W to about 1000W, and for such lamps that use a metal coil for both ignition and containment.
In said co-pending application Ser. No. 09/851,443, HID lamps of the ceramic metal halide type with power ranges of about 150W to about 1000W are provided that use a metal coil wound around the arc tube of such lamps for both ignition and containment. The nominal voltage range for 150W–400W lamp types is 100V–135V, and the nominal voltage range for 1000W lamps is 250–263V. Such constructions provide numerous benefits over the prior art that were not recognized or previously achieved.
The present invention provides still further improvements in said lamps. For example, over the life of the lamp, the coiled antenna is constantly exposed in a temperature environment of above 1200 degrees C. which may tend to decrease the effectiveness of the coiled antenna as a starting aid and as a containment aid. There is a need to insure that the effectiveness of such coiled antennae is maintained while exposed to high temperature environments.